Investment casting of metals



United States Patent INVESTMENT CASTING 0F METALS Hyman Rosenthal and Samuel Lipson, Philadelphia, Pa., assignors to the United States of America as represented by the Secretary of the Army No Drawing. Application May 27, 1952,

erial No. 290,375

2 Claims. (Cl. 106-38.35) (Granted under Tifle 35, U. S. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

The present invention relates to metal casting, and more partiiularly to an improvement in the art of investment casting of metals.

In the process known as investment casting, a pattern of the finished product is first prepared from a suitable material such as wax. A plaster-bonded silica slurry or other appropriate refractory mixture is next disposed about the wax pattern and permitted to set thereby forming a mold of refractory material, also referred to as an investment. After the investment is set, the wax pattern is removed by methods well known in the art and molten metal is then introduced into the cavity left by the pattern thereby to form the finished product. For one reason or another, as will become apparent from the description hereafter, certain surfaced defects in the finished product are encountered as a consequence of preparation of the investment.

It is, therefore, a primary object of the present invention to provide an improved in the process of preparing an investment which will reduce surface defects in the finished casting characteristic of prior art methods of preparation.

Another object of the present invention is to provide an improved slurry mixture useful in preparing an investment which will be effective in turning out a better finished casting.

PROBLEMS TO BE SOLVED Preparation of the investment may follow either one of two processes:

Process A The most common and simplest method involves a single coat process. In this process, the pattern is supported within a flask or suitable container and the slurry is poured or otherwise introduced into the container in a manner to surround the pattern. Once the slurry is set, the pattern is removed leaving a mold or investment having a cavity the shape of the finished casting.

The slurry mixture most frequently used for all but the highest melting alloys and which is considered to be most practicable for preparation of this type of investment comprises an aqueous slurry of a plaster-bonded silica mixture. For example, a standard slurry used for nonferrous castings comprises 60 cc. water to 100 grams of investment material (a silica and plaster mixture). Although this slurry is suitable for many applications, experience teaches that, as in most aqueous slurries, settling of the solid material occurs before the investment is set. The result of this condition is the collection of free water or slurry deficient in solid matter on the undersides of a pattern such that, in the finished casting, defects such as ridges or raised areas are formed on the surface. It is well known among persons skilled in the art that this condition may be partly alleviated by reducing the amount of Water in the slurry. However, this 'too is Patented Aug. 14, 1956 settling of the solid material in the mixture before setting of the investment.

A further problem to be solved by the present invention is to provide an improved slurry mixture for use as a single coat in preparing an investment which will effectively reduce the collection of free water before setting of the investment.

Process B The other process, referred to as a precoat process, is found to be particularly desirable for certain applications. In this process, the investment cavity is provided with a lining, generally referred to as a precoat, the characteristics of which are different from the characteristics of the investment backing material. In preparing an investment by this process, the pattern is first dipped in a slurry suitable to provide a relatively thin precoat. After setting of the precoat, a suitable backing material is applied around the precoat. Thus, with this type of investment, the precoat provides a thin barrier between the casting surface and the backing while at the same time taking advantage of the desirable characteristics of the backing material.

In preparing an investment by the precoat process, a standard slurry mixture, such as that described above, is generally used since it possesses the necessary characteristics to be desired in a precoat. However, certain difiiculties are encountered in its application. For example, the precoat material tends to run off the pattern before setting. This leaves exposed or and the material also has a tendency to collect in a fairly heavy layer at the lower surfaces of the pattern. In other words, the resulting precoat is not of the desired uniform thickness throughout.

Accordingly, a problem to be solved by the present invention is to provide an improvement in the process of preparing a precoat for an investment which will be free from the above and other disadvantages of prior art preparation methods.

Still another problem to be solved by the present invention is to provide an improved slurry mixture for use as a precoat in preparing an investment which will effectively reduce mobility of the mixture before setting of the precoat.

A further problem to be solved by tion is to provide an improved slurry preparing a precoat of substantially the present invenmixture for use in uniform thickness.

SOLUTION OF THE PROBLEMS thin areas in the precoat positions incorporating agar were investigated. The slurry mixture using 45 cc. H2O/100 grams solid investment material corresponds to the mix currently being used for investments for nonferrous castings. This mixture has been found excellent for providing both a single coat investment and a precoat if the problems of collection of free Water and mobility of the slurry or slip can be solved.

Process In accordance with the present invention, the process of preparing a slurry mixture is characterized by the ad ditional step of adding agar to the mixing liquid just prior to addition of the solid refractory ingredients. The addition of agar should, of course, be commensurate with the particular mixtures desired, such, for example, as the quantities mentioned hereafter.

Composition The most satisfactory mix from the point of view of ease of handling and which, for universal application, provides the best results may be made as follows: a 0.1 per cent aqueous solution of agar is made first; approximately 50 to 60 cc. of this solution is then mixed with about 100 grams of solid investment material (for example, silica and plaster). The percentage of agar in water assumes the condition that the mixing water is at room temperature (70 degrees Fahrenheit).

Although the proportion of ingredients may be varied somewhat to satisfy a particular condition, it has been found that the quantity of agar should be within the range 0.05 to 1.0 per cent by weight in the mixing liquid.

For a particular process, such as the single coat method, Process A above-mentioned, it is recommended that a slurry containing 0.1 to 0.2 per cent agar in the mixing water be used. The addition of this amount of agar is sufficient to effectively reduce surface defects caused by separation of free water before setting of the investment while at the same time preserving the advantageous characteristics of the standard slurry. The result will be found to provide a surface uniform smoothness with the characteristic surface defects substantially reduced if not wholly eliminated.

For use particularly as a precoat, as in Process B above-mentioned, it is recommended that a slurry containing 0.2 per cent agar in the mixing water be used. This mixture is effective in reducing mobility of the investment during the setting process When it is used as a dipping precoat without destroying the qualities of the standard slurry mixture. Use of such a mixture in preparation of plaster-bonded silica precoats Was found to be 4 effective in reducing mobility of the slurry and a precoat of uniform thickness resulted, a characteristic highly desirable in a precoat.

SUMMARY From the foregoing, it is apparent that, as a result of the present invention, substantially improved results are obtainable with the addition of agar to a slurry mixture. At the same time, it will become apparent to those persons skilled in the art that, to facilitate preparation of the slurry mixture, a stock solution of agar can be prepared in advance. The stock solution should, however, be maintained at approximately 50 degrees Centigrade temperature until used in order to avoid gel formation. A convenient stock solution is that containing one per cent agar. This should not be added to the mixing water of the slurry until just prior to addition of the solid material.

It should be understood that, while the present invention has been described with particular reference to the standard plaster-bonded silica slurry, it may be applicable to other refractory mixtures within the spirit of the invention. It will also be recognized by those persons skilled in the art that the characteristics of agar are not always uniform. In the latter event, minor adjustments in composition of materials should be permitted within the spirit of the invention to compensate for these variations. Therefore, it is desired that the foregoing description be considered merely as illustrative and not as limiting.

We claim:

1. In a slurry of a refractory material for preparing an investment, said slurry consisting essentially of a plasterbonded silica mixture having incorporated therein a concentration of 0.05 to 1.0 per cent agar by weight in the mixing liquid.

2. In a slurry of a refractory material for preparing an investment, said slurry consisting essentially of a plaster-bonded silica mixture having incorporated therein a concentration of approximately one-tenth of one per cent agar by weight in the mixing liquid.

References Cited in the file of this patent UNITED STATES PATENTS 814,007 Ams Mar. 6, 1906 1,724,134 Iredell Aug. 13, 1929 1,901,052 Dailey Mar. 14, 1933 2,345,211 Neiman Mar. 28, 1944 2,422,497 Noyes June 17, 1947 2,491,096 Feagin Dec. 13, 1949 

1. IN A SLURRY OF A REFRACTORY MATERIAL FOR PREPARING AN INVESTMENT, SAID SLURRY CONSISTING ESSENTIALLY OF A PLASTERBONDED SILICA MIXTURE HAVING INCORPORATED THEREIN A CONCENTRATION OF 0.05 TO 1.0 PER CENT AGAR BY WEIGHT IN THE MIXING LIQUID. 